﻿Isodonxiaoluzhiensis (Lamiaceae, Nepetoideae), a new species from Yunnan, southwest China

﻿Abstract Isodonxiaoluzhiensis, a new species of the tribe Ocimeae in family Lamiaceae, is described and illustrated. The new species is known only from the type locality, Xiaoluzhi village in Luzhijang dry-hot valley of Yimen County, central Yunnan, southwest China. It is characterized by having a procumbent habit, gracile stems and branches, relatively small leaves and flowers, and the phenology of flowering in winter. The morphological comparisons with its putative closest relatives (I.adenanthus and I.hsiwenii) are also presented.

China possesses a rich set of species of Isodon, and the center of species diversity of the genus was found in southwest China, especially in the Hengduan Mountains region (Li 1988;Li and Hedge 1994;Yu et al. 2014).The first comprehensive revision of this genus in China was conducted by Wu and Li (1977) for the "Flora Reipublicae Popularis Sinicae", in which the generic name Rabdosia.was applied instead of Isodon.Wu and Li (1977) recognized 90 species and 21 varieties in China, and divided these Chinese species into four sections, namely Isodon sect.pyramidium (Benth.)H. W. Li, I. sect.amethystoides (Benth.)H. W. Li, I. sect.Isodon, and I. sect.Melissoides (Benth.)H. W. Li, and the section Isodon was further divided into 10 series.Li and Hedge (1994) reviewed the Chinese species in the Flora of China, recognized 77 species in China, 64 being endemics.Recently taxonomic novelties of this genus have been consistently reported from China (Xiang and Liu 2012;Chen et al. 2014Chen et al. , 2016Chen et al. , 2017Chen et al. , 2019Chen et al. , 2021)).
In January 2018, during our botanical fieldwork to the Luzhijiang River valley at Yimen County, Yunnan, southwest China, an unknown plant of Isodon was encountered and gathered.In 25 September 2021, the same plant was discovered again at the same site.Based on critical comparison with related species, it was confirmed that this plant represents a distinct new species which is described here.

Materials and methods
The study followed the normal practice of plant taxonomic survey and herbarium taxonomy.Morphological studies of the new species were based on observation of living plants and specimens housed at YUKU.Digital images of type specimens of genus Isodon available at the JSTOR Global Plants (http://plants.jstor.org/),as well as collections housed at CDBI, KUN, PE, PYU and YUKU, were extensively examined and compared with the new species.Pertinent taxonomic literature (Wu and Li 1977;Li 1988;Xiang and Liu 2012;Chen et al. 2014Chen et al. , 2016Chen et al. , 2017Chen et al. , 2019Chen et al. , 2021) ) was extensively consulted.Measurements were carried out under a stereomicroscope (Olympus SZX2, Tokyo, Japan) using a ruler and a metric vernier caliper.Diagnosis.Isodon xiaoluzhiensis is most similar to I. adenanthus (Diels) Kudô in having similar flower shape, but it clearly differs from the latter in its procumbent (vs.erect or ascending in I. adenanthus) habit, stems and branches woody (vs.non-woody) with densely white glandular puberulent (vs.densely retrorse gray pubescent), leaves usually narrowly ovate to rhomboid, rarely lanceolate (vs.rhombic-ovate to ovate-lanceolate), small, 0.8-1.4cm long (vs.1.5-6.5 cm long), 0.2-0.5 cm wide (vs.1.0-2.5 cm wide), teeth of calyx subobtose to subacute (vs.apiculate) at apex, posterior lip of corolla non-spotted (vs.purple spotted).Additionally, I. xiaoluzhiensis flowers from November to January, nevertheless I. adenanthus usually flowers from March to August.
Phenology.Flowering from November to January, fruiting from December to February.
Etymology.The specific epithet "xiaoluzhiensis" is derived from the type locality of the new species, the Xiaoluzhi village, and the Latin suffix -ensis, indicating the place of origin or growth.
Preliminary conservation status.Isodon xiaoluzhiensis is a rare species with a restricted distribution and small population size.It is only known from in the upstream region of the Luzhijiang River in the Yimen county, which is no protected area covering.The total population size is estimated at less than 50.According to the IUCN Standards and Petitions Subcommittee (2019), this new species should be considered as "Critically Endangered" (CR).
Discussion.Isodon xiaoluzhiensis exhibits all the characteristics of Isodon, but having a procumbent habit, relatively small leaves, and the phenology of flowering in winter can easily differentiate it from other species in the genus.Especially, the procumbent habit is rare in the entire genus Isodon, just I.  very similar to I. adenanthus (Diels) Kudô in terms of flower shape and size, but it clearly differs from the latter by its stem woody (vs.non-woody), procumbent (vs.erect or ascending), highly (vs.sparsely) branched, densely white glandular puberulent (vs.densely retrorse gray pubescent), leaves usually narrowly ovate to rhomboid (vs.rhombic-ovate to ovate-lanceolate), papery or thinly coriaceous (vs.herbaceous), small, 0.8-1.4cm long (vs.1.5-6.5 cm long), 0.2-0.5 cm wide (vs.1-2.5 cm wide), teeth of calyx subobtuse to subacute (vs.apiculate) at apex, posterior lip of corolla non-spotted (vs.purple spotted).
Isodon xiaoluzhiensis is somewhat close to I. hsiwenii Y. P. Chen et C. L. Xiang in sharing relatively small leaves and procumbent stems.However, I. xiaoluzhiensis diffeers from I. hsiwenii by its main stems up to 60 cm long (vs. up to 100 cm for I. hsiwenii), leaves adaxially green or purplish black with pellucid glands (vs.dark green, densely puberulent and colorless glandular), leaves abaxially gray-green and densely white glandular-puberulent (vs.light green, densely puberulent colorless glandular on both sides), calyx purple with few green (vs.green outside), veins densely white hirsute outside (vs.densely purplish puberulent on veins), calyces teeth at apex subobtuse to subacute (vs.acute).Additionally, the habitats of these two species are distinctly different and non-overlapping.Isodon hsiwenii is only known from northeast Yunnan, situated in Jinshajiang River basin, and grows on stony slopes at an altitude of approximately 1 750 meters.Conversely, I. xiaoluzhiensis is discovered in Central Yunnan, located within the Honghe River basin, and inhabits the limestone grasslands between 1300 m and 1400 m at elevation.A morphological comparison of I. xiaoluzhiensis with I. adenanthus and I. hsiwenii is provided in Table 1.

Figure 1 .
Figure 1.Isodon xiaoluzhiensis sp.nov.A habit B flower (side view) C calyx D corolla (vertical view) E style and stamens.
Distribution and ecology.According to the present investigations, I. xiaoluzhiensis is only found in its type locality, i.e.Xiaoluzhi village of Luzhijiang valley, Yimen County, Yunnan Province, southwest China, where the climate is seasonally hot and arid.Isodon xiaoluzhiensis occurs in the limestone grasslands between 1,300 m and 1,400 m elevation.In the type locality, its association mainly includes Dodonaea viscosa (L.) Jacq.(Sapindaceae), Indigofera vallicola Huan C.Wang et Jin L. Liu (Leguminosae) (a new species discovered by Liu et al. (2022)), Duhaldea lachnocephala Huan C. Wang et Feng Yang (Compositae) (an endemic species of Luzhijiang valley discovered by Yang et al. (

Figure 3 .
Figure 3. Isodon xiaoluzhiensis sp.nov.A, B habit C perennial stem D branchlet E adaxial surface of leaf F abaxial surface of leaf G-I inflorescence J corolla (front view) K flower (lateral view) L corolla (rear view) M calyx.

Figure 4 .
Figure 4. Isodon xiaoluzhiensis sp.nov.A adaxial surface of leaf B abaxial surface of leaf C flower (lateral view) D flower without calyx E calyx F pistil and stamens.